Cyclic adsorption processes are frequently used to separate the components of a gas mixture. Typically, cyclic adsorption processes are conducted in one or more adsorbent vessels that are packed with a particulate adsorbent material which adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out. For example, PSA processes generally comprise a series of steps that includes pressurization of the adsorbent-containing vessel to the desired adsorption pressure, production of purified gas by passing feed gas through the adsorption vessel at the desired adsorption pressure and regeneration of the adsorbent when the concentration of adsorbed component(s) in the adsorbent reaches a certain level.
The procedure followed for regenerating the adsorbent varies according to the process. In PSA processes the adsorbent is at least partially regenerated by reducing the pressure in the adsorption vessel, thereby causing adsorbed component to be desorbed from the adsorbent, whereas in (TSA) processes the adsorbent is regenerated by heating it, thereby causing the adsorbed component to be desorbed. In either case, the regeneration process usually includes a purge step during which a gas stream that is depleted in the component to be desorbed is passed countercurrently through the bed of adsorbent, thereby reducing the partial pressure of adsorbed component in the adsorption vessel, which causes additional adsorbed component to be desorbed from the adsorbent. The nonadsorbed gas product is generally used to purge the adsorbent beds, since this gas is usually quite depleted in the adsorbed component of the feed gas mixture. It often requires a very considerable quantity of purge gas to adequately regenerate the adsorbent. For example, it is not unusual to use half of the nonadsorbed product gas produced during the previous production step to restore the adsorbent to the desired extent.
It can be appreciated that using such large quantities of nonadsorbed product gas to regenerate the adsorbent detracts considerably from the efficiency of the adsorption process. Process modifications which increase the efficiency of adsorption processes are continually sought. This invention provides a process modification which significantly increases the net production of nonadsorbed product gas in PSA and TSA processes.